Tuning the emissive triplet excited states of platinum(ii) Schiff base complexes with pyrene, and application for luminescent oxygen sensing and triplet–triplet-annihilation based upconversions

Abstract

Pt(II) Schiff base complexes containing pyrene subunits were prepared using the chemistry-on-complex approach. This is the first time that supramolecular photochemical approach has been used to tune the photophysical properties of Schiff base Pt(II) complexes, such as emission wavelength and lifetimes. The complexes show intense absorption in the visible region (ε = 13100 M⁻¹ cm⁻¹ at 534 nm) and red phosphorescence at room temperature. Notably, much longer triplet excited state lifetimes (τ = 21.0 μs) were observed, compared to the model complexes (τ = 4.4 μs). The extension of triplet excited state lifetimes is attributed to the establishment of equilibrium between the metal-to-ligand charge-transfer (³MLCT) state (coordination centre localized) and the intraligand (³IL) state (pyrene localized), or population of the long-lived ³IL triplet excited state. These assignments were fully rationalized by nanosecond time-resolved difference absorption spectra, 77 K emission spectra and density functional theory calculations. The complexes were used as triplet sensitizers for triplet–triplet-energy-tranfer (TTET) processes, i.e. luminescent O₂ sensing and triplet–triplet annihilation (TTA) based upconversion. The O₂ sensitivity (Stern–Volmer quenching constant) of the complexes was quantitatively evaluated in polymer films. The results show that the O₂ sensing sensitivity of the pyrene containing complex (K_SV = 0.04623 Torr⁻¹) is 15-fold of the model complex (K_SV = 0.00313 Torr⁻¹). Furthermore, significant TTA upconversion (upconversion quantum yield Φ_UC = 17.7% and the anti-Stokes shift is 0.77 eV) was observed with pyrene containing complexes being used as triplet sensitizers. Our approach to tune the triplet excited states of Pt(II) Schiff base complexes will be useful for the design of phosphorescent transition metal complexes and their applications in light-harvesting, photovoltaics, luminescent O₂ sensing and upconversion, etc.